The production and use of fossil fuels contribute to an increase in emissions of greenhouse gases (GHGs), especially carbon dioxide (CO2) and other pollutants such as oxides of sulfur (SOx) and oxides of nitrogen (NOx). In Canada, CO2 constitutes the largest fraction of greenhouse gas emissions, accounting for about 80% of the total greenhouse gases emitted. Besides its greenhouse effects, CO2 is also blamed for climate changes and global warming. Through ratification of the Kyoto protocol, Canada is committed to cap greenhouse gas emissions by 6% below the 1990 level. To achieve this target, there needs to be a reduction of about 39% of greenhouse gas emissions from the projected levels by 2010 or about 240 million tonnes of CO2. As a result, large point sources of CO2 emissions such as coal-fired power plants, refineries, cement manufacturers and the like need to be monitored and stringently regulated.
Although captured CO2 can be used in a wide variety of industrial applications, such as in the enhanced oil recovery processes, in which recovered CO2 can be used to produce more oil from petroleum reservoirs while part of the CO2 is simultaneously sequestered in the reservoir, as well as in the manufacturing of commodity chemicals, in which recovered CO2 can be used as a potential chemical feedstock, the process of capturing CO2 efficiently from gas streams is difficult to perform. Thus, intensive research efforts have been made in recent years to develop methods for recovering the CO2 emitted from industrial gas streams and for storing the recovered CO2 without discharging it into the atmosphere.
Conventionally, to reach a desired target for CO2 capture, aqueous alkanolamine solutions have been used to absorb CO2 from low-pressure streams such as flue gases emitted from power plants. A commonly used alkanolamine is monoethanolamine, MEA. From a structural standpoint, one of the advantages of using alkanolamines is that they contain at least one hydroxyl group, which helps to reduce the vapor pressures of alkanolamines and thus minimize the losses of the product during hot regeneration. Another advantage of using alkanolamines is that the presence of the hydroxyl group increases the solubility of the alkanolamines in aqueous solutions, thus allowing the use of highly concentrated absorbing solutions. Yet another advantage of using alkanolamines is that the presence of the amino group provides the necessary alkalinity to absorb CO2 (Kohl, A. L. and Reisenfeld, F. C., Gas Purification, 4th ed., Gulf Publishing Co., Houston, Tex., 1985; Kohl, A. L. and Nielsen, R. B., Gas Purification, 5th ed., Gulf Publishing Co., Houston, Tex., 1997). Thus, for over 70 years, alkanolamines have long been the solvent of choice for CO2 removal on a commercial scale. In fact, aqueous alkanolamine solutions are the most widely used solvents for CO2 and H2S absorption.
For many years, the basic alkanolamine process for CO2 capture has remained unchanged but current demands to reduce energy consumption, decrease solvent losses and improve air and water qualities have resulted in several modifications to upgrade the process. The most important improvement is the introduction of specially formulated solvents. Depending on the process requirements, for example, selective removal of H2S and/or CO2-bulk removal, several options for alkanolamine-based treating solvents with varying compositions are available.
Recently, some companies have developed proprietary hindered amines for use in removing acid gases from liquid and gas streams. A new class of amines, in particular sterically hindered amines, has thus been introduced as commercially attractive amines. These hindered amines can be either primary such as 2-amino-2-methyl-1-propanol (AMP), or secondary, such as diisopropanolamine (DIPA), and they have been found to require much less energy for regeneration than conventional alkanolamines. Accordingly, these hindered amines are useful as promoters and as components of organic physical solvent/amine systems. Furthermore, they have been found to be useful in selective absorption of H2S in the presence of CO2 (Sartori, G. and Leder, F., U.S. Pat. No. 4,112,050; Sartori, G. and Leder, F., U.S. Pat. No. 4,112,051; and Sartori, G. and Leder, F., U.S. Pat. No. 4,112,052).
In addition to the sterically hindered amines described above, some companies have developed formulated amines. Generally, formulated amines are broadly defined as amines that have been specifically formulated to perform a specific task, for example, selective separation of H2S from light hydrocarbons in the presence of CO2, bulk separation of CO2, etc. (Chakma, A., “Separation of Acid Gases from Power Plant Flue Gas Streams by Formulated Amines”, Separation Technology, Vol. 11, pp. 727-737, 1994; and Chakma, A., “Formulated Solvents: New Opportunities for Energy Efficient Separation of Acid Gases”, Energy Sources, Vol. 21, pp. 51-62, 1999). A formulated amine may consist of a single solvent such as 3-(dimethylamino)-1,2-propanediol (DMAPD) (Iijima, M. and Mitsuoka, S., U.S. Pat. No. 5,736,115) or 2-(diethylamino)-ethanol (DEAE) (Yoshida, K., Mimura, T., Shimojo, S., Karasaki, M., Iijima, M., Seto, T. and Mitsuoka, S., U.S. Pat. No. 6,500,397) or a solvent mixture such as a mixture of modified polyamines with formaldehyde or with formaldehyde and phenol (Rinaldi, G., “Acid Gas Absorption by Means of Aqueous Solutions of Regenerable Phenol-Modified Polyalkylenepolyamines”, Ind. Eng. Chem. Res., Vol. 36, pp. 3778-3782, 1997; and Filippis, P. D., Giavarini, C., Maggi, C., Rinaldi, G. and Silla, R., “Modified Polyamines for CO2 absorption. Production, Preparation and Characterization”, Ind. Eng. Chem. Res., Vol. 39, pp. 1364-1368, 2000) in aqueous solution. Most of the proprietary solvents marketed by the major solvent manufacturers for CO2 capture are based on formulated amines. By judicious choice of a formulated amine or an amine mixture, the process efficiency of removing acid gases from liquid and gas streams can be enhanced significantly as compared to the use of traditional amines. Furthermore, some of the gas processing problems that cannot be dealt with using the conventional technology in an economical manner can be easily handled with formulated amines.
While it is possible to obtain a cost reduction in CO2 capture using formulated alkanolamines, this route may not necessarily present the most optimum scenario for the process. Further, since the amines which are required for formulation are typically those that are commercially available, the scope for optimization is thereby limiting to existing amines.
There is a need for developing an efficient and cost effective method for capturing or removing CO2 from gas streams.